The problem of purifying liquids such as water is complicated by the fact that liquids can be contaminated by a variety of agents and removal of each agent requires a special technique. For example, particulates may be removed by settling techniques. Liquid organic contaminants may be removed by flushing water through a bed of activated carbon. Negative ions such as chlorine are removed by flushing the water through a "strong base" ion exchange resin whereby the resin gives up a hydroxyl radical and absorbs the negative ion. Ions of heavy metals or calcium are removed by flushing the water through a bed of "strong acid" ion exchange resin whereby the resin gives up a hydrogen ion and absorbs the metal ion. Another method of removing heavy metal ions is to raise the pH in order to precipitate heavy metal oxides.
A method developed by the Unocal Corporation, called the UNIPURE PROCESS, does not depend on raising the pH of polluted waters in order to precipitate the heavy metal ions. The process is based on a technique of precipitating iron in an activated state that absorbs heavy metal ions An agent is then added to the water to flocculate the precipitate. Finally, the flocculant is separated from the pure water, usually by settling in a settling tank.
An important problem associated with the addition of precipitating agents (such as lime, ammonia or oxygen) to form a floc is the generation of gases that form bubbles in the sludge. Since the general approach to removing the sludge is by settling, the presence of bubbles greatly impedes the settling since the bubbles are trapped by the flocs and remain suspended or actually rise instead of settle.
Many water purification systems comprise separate components such as a settling tank followed by a bank of cylinders containing activated carbon and then finally a bank of cylinders containing the resins. These systems typically require a lot of space and are expensive to build and maintain in as much as a considerable amount of plumbing is involved. In many operations, considerable expense has already been invested in tanks, etc. which deter the owner from investing in improved equipment.
It is known in the art to force liquid to rise through an inclined tube in order to accelerate the settling of particles suspended in the liquid to the bottom of the tube It is found that by inclining the tube at an appropriate angle, the rate of settling is greatly accelerated The most effective angle can vary somewhat, depending on current (rate of flow) and nature of the suspended particles (particle size, state of agglomeration, etc.). It has been found that the most effective angle of inclination is about 50.degree..
If a settling tank is constructed with straight tubes in an erect cylindrical or rectangular tank, the requirement for inclining the tubes results in there being a substantial amount of the tank space and surface not being used. Therefore, tanks have been built with slanted sides although this construction is expensive and requires considerable space. The flow of the fluid through the tube is laminar, i.e., the velocity of the stream varies with distance from the lowermost inner surface of the tube. The more dense layer of liquid is on the lowermost side of the tube where particles have accumulated so that liquid actually flows downward carrying the particles down the incline while cleaner, less dense liquid flows upward and along toward the top most side of the tube.
U.S. Pat. No. 4,589,984 to Legrand et al discloses a settling tank for separating oil, water and sludge comprising a parallel connection of settling regions, each region having a sloped bottom for particulate separation under a bundle of vertical tubes to separate the oil from water.
U.S. Pat. No. 4,221,671 to Meurer discloses a construction of strips of corrugated sheet, where the corrugations are parallel to the direction of flow. Each strip comprises a series of short straight corrugated sections connected end to end to form a helix The helices are interleaved with one another so as to fill most of a cylindrical container.
The Meurer patent also discusses a settling tank comprising an array of tubes wound helically around a central stand pipe in which the tubes are arranged as concentric circular layers, where the diameter of the tubes of each outer layer is successively larger than the tubes of the next inner layer. It is pointed out that each layer of tubes would have different flow characteristics than neighboring layers The same patent discloses layers of helically wound tubes having the same diameter but states that "the problem of having to curve the tubes differently in each row is manifestly impractical".
U.S. Pat. No. 3,613,889 to Reed discloses a system where bundles of inclined tubes are floated on the top of the surface of the tank so that changes of the liquid level in the tank can be accommodated.
U.S. Pat. No. 4,783,255 to Bogusch discloses a settling device comprising straight channels where each channel has a cross-section resembling a boomerang.